Gels were made from independent root tip preparations from three different batches of plants. In-gel digestion, sample preparation, MALDI TOF and MALDI TOF-TOF peptide mass fingerprint and database searching was carried out as described in detail elsewhere. Statistical significance was tested with a t-Student test . A 2-fold change in spot signal intensity between treatments was taken as a threshold.Metabolite extraction and analysis was carried out as described previously. Root tips from eight different replicates per treatment were used. After metabolite extraction and derivatization, samples were injected randomly in split-less mode with a cold injection system and analyzed by GC using a Rtx 5Sil MS column and an integrated guard column . The GC was connected to a Leco Pegasus IV TOFMS spectrometer controlled with Leco ChromaTOF software v.2.32 . Initial peak detection and mass spectra deconvolution were performed with Leco Chroma-TOF software v.2.25. GC-MS chromatograms were processed as described previously. Further analysis after deconvolution was done using the semi-automated workflow in the UC Davis Genome Center metabolomics laboratory. Metabolite data were normalized using FW and the sum of all metabolite heights in a single run to account for small FW and GC injection variations. Statistical analysis, including i) breakdown one-way ANOVA univariate statistics , ii) multivariate analysis supervised partial least square and iii) unsupervised principal component analysis were carried out with Statistica software .
A 4-fold change in signal intensity between treatments was taken as a threshold for discussion.Crop production, especially in arid and semi-arid regions of the world where rainfall is not able to meet the evapotranspiration needs of the crops,greenhouse vertical farming depends on supplemental irrigation. Irrigated agriculture contributes 40% of the world food production from 20% of the cropped area, thus makes a major contribution to the global food security . However, irrigated agriculture may become unsustainable due to its contribution to soil degradation, salinization, waterlogging, and environmental pollution. Global water security warrants beneficial reuse of recycled water, such as irrigation, but with minimal potential harmful impacts on ecosystems. Ecosystem impairment, particularly reduced soil quality, biodiversity loss, and harm to amenity and cultural heritage values, is a growing global problem . Therefore, future irrigation schemes must address trade-offs, particularly with respect to inter-sectoral water allocations and environmental impacts. Aquatic ecosystems adjacent to irrigated agriculture are most at risk due to the transport of irrigation induced chemicals such as soluble salts, nitrates, and pesticides . The fate of these chemicals in the soils and their migration to receiving environments depend on a number of factors including the vegetation, topography, climate, soil, irrigation, groundwater level, and flow conditions in the stream . Riparian vegetation can moderate the movement of water and solutes to water bodies by interception and attenuation of chemicals moving through the buffer zone . Several investigations have examined the functions of buffer zones for stream ecosystems . However, these have mostly dealt with the overland movement of solutes via surface runoff and sediment transport.
Only, a limited number of modelling or case studies have evaluated the role played by buffer zones in reducing the migration of irrigation induced soluble salts/ contaminants via subsurface flow to streams . Subsurface flow paths can exhibit wide variations depending on specific local conditions including subsurface lithology and stratigraphy . To our knowledge, no information is currently available in the literature on the role played by buffer zones in dealing with the irrigation induced solute interception or influencing its migration to water bodies. Field experiments for assessing the role of a buffer zone on the subsurface water and salts movement from irrigated cropping system to an adjoining river is both a complex and expensive exercise. Therefore, numerical models are increasingly being used for such assessment. Hydraulic exchange across the stream-aquifer has been modelled with buffer zones or without . Similarly, Kidmose et al. employed a conceptual groundwater flow and reactive transfer model to establish a relationship between flow paths and the fate of a pesticide in a riparian wetland. Alaghmand et al. used a numerical model to evaluate the interaction between a river and a saline floodplain in relation to groundwater fluctuations, incorporating evapotranspiration losses by riparian vegetation. Klatt et al. explored the capability of a coupled hydro-biogeochemical model to evaluate the effectiveness of buffer strips to reduce nitrogen loads into aquatic systems. However, most of these modelling studies have been either conceptual and/or only partially calibrated for site specific flow and/or solute dynamics. The complicated nature of water and solute transport processes and the inherent uncertainty of input data are some of the challenges in simulating water flow and solute transport with physically based models.
Nevertheless, such models are valuable in understanding water flow and solute transport/reaction processes involved in complex bio-geological environments. This study uses a two-dimensional finite element numerical model HYDRUS to quantify the extent of water and solute exchange across a stream buffer interface. The study involves complex heterogeneous geological formations involving real-time climatic, vegetative , and stream flow conditions. The key objectives of this investigation were: i) to calibrate and validate a numerical model for water table dynamics in an area adjacent to a seasonal river by incorporating daily water level fluctuations in the river, groundwater dynamics, crop evapotranspiration, riparian zone vegetation evapotranspiration, and soil heterogeneities; ii) to estimate the impact of different buffer zone widths on the flux exchange at the river buffer interface under different cropping systems, iii) to optimize the riparian width to control the irrigation-induced solute movement to the river for different irrigated crops; and iv) to estimate the residence time of the solute tracer migrating to the adjoining water body through the subsurface under shallow water table conditions.The study was carried out at the Virginia Park gauging station at Gawler River which is situated at 12 m above the Australian Height Datum . The Gawler River only flows during the rainy season . However,vertical agriculture stagnant water /base flow conditions prevail at other times at the gauging station. The adjacent area, being a part of the vast Northern Adelaide Plains , has a relatively flat topography with a gentle slope to the west. All relevant features of the study site are shown in Fig. 1. The NAP experiences a Mediterranean climate, which is characterised by hot, dry summers and cool to cold winters. Long-term average rainfall in the region amounts to 475 mm and annual evapotranspiration amounts to 1308 mm, resulting in the irrigation demand for crop production. Water table fluctuations in the area adjacent to the river were monitored in the shallow wells . Location of these wells is shown in Fig. 1.The soils of the NAP are highly heterogeneous with depth. There is commonly a shallow clay layer at a variable depth, which determines the root growth and crops to be grown. Broader soil groups and geology of the site were obtained from the stratigraphic information of the site and well logs within the vicinity of the site. There are in general 6 major geological layers, which include red friable sandy loam soil, light brown silty topsoil, sandy clay, sandy non-calcareous clay, non-calcareous fine sandy clay, and sand. The soil particle size distributions and bulk densities of these soil groups were obtained from the previous soil analysis reported in ASRIS and the APSIM data base. The particle size and bulk density data were used to estimate soil hydraulic parameters using the ROSETTA module embedded in the HYDRUS software environment. The saturated hydraulic conductivity , and the α and n parameters were further adjusted during the calibration process and their final optimized values are presented in Table 1.must be noted that root water uptake in HYDRUS depends on the availability of water in the soil, the root spatial distribution, and differential transpiration fluxes in the crop and buffer zones. The root water uptake was assumed to be linearly distributed with depth, with the maximum at the soil surface and zero at the bottom of the rooting zone. The “trigger irrigation” option was used to generate irrigation schedules for all crops .
The trigger pressure used for wine grapes, almond, and carrot-potato, respectively, were −60, −25, and −15 kPa at a depth of 30 cm. Similar trigger pressures have been used for these crops in previous studies .The transport domain represents a 400 m cross section from the middle of the river . The vertical dimension represents the distance from the Australian Height Datum to the soil surface at the experimental site. The top width of the river was 10 m, the bottom width 4 m, and the depth 4 m at the study site. The width of the buffer zone is 30 m from the river bank. Therefore, the lateral width of the riparian zone at the Virginia Park gauging station from the middle of the river is approximately 35 m, which also includes an unsealed road which runs along the river. The finite element discretization resulted in 10,000 2D elements in a standard rectangular 2D domain. On the upper left side of the domain , the atmospheric boundary was considered through which the infiltrative influx or the evapotranspirative efflux occurs. A time-variable flux boundary condition was imposed on the upper right side of the domain to represent the buffer zone, which had different fluxes than the irrigated surface. The flux at this boundary was given by the difference between daily rainfall and daily potential evaporation . A special HYDRUS boundary condition was specified in the river. This special BC assigns the hydrostatic pressure head on the boundary below the water level in the river and a seepage face BC on the boundary above the water level. The specified water levels in the river are linearly interpolated in time in order to smooth the impact of daily fluctuations of water levels in the river . Measured values of water table depths in the well near the left boundary of the domain were used to define initial and time-variable pressure head boundary conditions. No flow was assumed as the boundary. The initial pressure head condition in the domain was specified by interpolating measured mean water table depths in the shallow wells in the adjacent area while considering hydrostatic equilibrium conditions in the vertical direction.The longitudinal dispersivity was assumed as one tenth of the modeling domain and the molecular diffusion coefficient in water equal to 1.66 cm2 /day .Measured water table depths in the shallow wells near the study site were used for the calibration and validation of the model. Simulations were carried out for 1461 days to calibrate the model for water table depths at the middle of the domain . For most sensitive model parameters including the saturated hydraulic conductivity , and the coefficients α and n of different soil layers were varied manually and no automated parameter optimization procedure was used to calibrate the model. In addition to a visual comparison of observed and simulated water table depths, a quantitative evaluation of the model performance was undertaken using goodness-of-fit measures similar to other studies . The calibrated model was validated for 1461 days by comparing the measured and simulated water table depths. The calibrated and validated model was then used to assess the impact of other irrigated crops and the buffer zone widths on the migration of water and solutes to the river. More details on different scenarios are given below in the Scenario Analysis section. To understand the movement of irrigation-induced solutes/agrochemical tracers to the river water, we considered Total Dissolved Solids as representative of all soluble solutes which is consistent with numerous studies . The initial soil conditions in the domain were assumed to be solute free. The average quantity of TDS of Class-A treated water from the Bolivar treatment plant was applied during all triggered irrigations at the atmospheric boundary where crop is being grown. However, the model calibration for solute dynamics could not be conducted due to the non-availability of site-specific data for solute transport processes.The calibrated and validated model was then used to simulate the dynamics of the hydrological fluxes and solute movement for different buffer widths and for various irrigated crops .